Polyester sulfone oligomers and blends

ABSTRACT

High performance polyester sulfone oligomers are prepared for aerospace applications by condensing mono- or difunctional crosslinkable end caps (i.e. unsaturated hydrocarbons having one or two crosslinking sites) with dicarboxylic acid halides and dialcobols (i.e. diols). Multidimensional oligomers have an aromatic hub from which the polyester chains radiate. Blends of the linear and multidimensional oligomers can be made using compatible, non-crosslinking polymers. Prepregs and composites are formed from the oligomers or blends.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application based upon U.S. patent application Ser. No. 726,259, filed Apr. 23, 1985, now abandoned.

TECHNICAL FIELD

The present invention relates to linear or multidimensional polyester oligomers and blends that include unsaturated hydrocarbon end caps to provide solvent resistance through crosslinking upon curing. Preferably the backbone includes electronegative ("sulfone") segments that alternate with the ester linkages.

BACKGROUND ART

Recently, chemists have sought to synthesize oligomers for high performance advanced composites suitable for aerospace applications. These composites should exhibit solvent resistance, be tough, impact resistant, and strength, be easy to process, and be thermoplastic. Oligomers and composites that have thermo-oxidative stability, and, accordingly can be used at elevated temperatures, are particularly desirable.

While epoxy-based composites are suitable for many applications, their brittle nature and susceptibility to degradation make them inadequate for many aerospace applications, especially those applications which require thermally stable, tough composites. Accordingly, research has recently focused upon polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness. The maximum use temperatures of conventional polyimide composites, such as PMR-15, are still only about 600°-625° F., since they have glass transition temperatures of about 690° F.

Linear polysulfone, polyether sulfone, polyester, and polyamide systems are also known, but each of these systems fails to provide as high thermal stability as is required in some aerospace applications.

There has been a progression of polyimide sulfone compounds synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode synthesized maleic-capped, linear polyarylimides as disclosed in U.S. Pat. No. 3,839,287. Holub and Evans synthesized maleic- or nadic-capped, imido-substituted polyester compositions as disclosed in U.S. Pat. No. 3,729,446. We synthesized thermally stable polysulfone oligomers as disclosed in U.S. Pat. No. 4,476,184 or U.S. Pat. No. 4,536,559, and have continued to make advances with polyetherimidesulfones, polybenzoxazolesulfones (i.e., heterocycles), polybutadienesulfones, and "star" or "star-burst" multidimensional oligomers. We have shown surprisingly high glass transition temperatures and desirable physical properties in many of these oligomers and their composites, without losing ease of processing.

Multidimensional oligomers, such as disclosed in our applications U.S. Ser. Nos. 726,258 (now abandoned); 810,817 (now abandoned); and 000,605, (now U.S. Pat. No. 5,210,213) are easier to process than many other advanced composite oligomers since they can be handled at lower temperatures. Upon curing, however, the unsaturated phenylimide end caps crosslink so that the thermal resistance of the resulting composite is markedly increased with only a minor loss of stiffness, matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties. Glass transition temperatures above 950° F. are achievable.

Commercial polyesters, when combined with well-known diluents, such as styrene, do not exhibit satisfactory thermal and oxidative resistance to be useful for aircraft or aerospace applications. Polyarylesters are unsatisfactory, also, since the resins often are semicrystalline which makes them insoluble in laminating solvents, intractable in fusion, and subject to shrinking or warping during composite fabrication. Those polyarylesters that are soluble in conventional laminating solvents remain so in composite form, thereby limiting their usefulness in structural composites. The high concentration of ester groups contributes to resin strength and tenacity, but also makes the resin susceptible to the damaging effects of water absorption. High moisture absorption by commercial polyesters can lead to distortion of the composite when it is loaded at elevated temperature.

High performance, aerospace, polyester advanced composites, however, can be prepared using crosslinkable, end-capped polyester imide ether sulfone oligomers that have an acceptable combination of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. By including Schiff base (--CH═N--), imidazole, thiazole, or oxazole linkages in the oligomer chain, the linear, advanced composites formed with polyester oligomers of our application U.S. Ser. No. 726,259 (now abandoned) can have semiconductive or conductive properties when appropriately doped.

Conductive and semiconductive plastics have been extensively studied (see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987; 4,344,869; and 4,344,870), but these polymers do not possess the blend of properties which are essential for aerospace applications. That is, the conductive polymers do not possess the blend of (1) toughness, (2) stiffness, (3) elasticity, (4) ease of processing, (5) impact resistance (and other matrix stress transfer capabilities), (6) retention of properties (over a broad range of temperatures), and (7) high temperature resistance that is desirable on aerospace advanced composites. These prior art composites are often too brittle.

Thermally stable multidimensional oligomers having semiconductive or conductive properties when doped with suitable dopants are also known and are described in our copending applications (including U.S. Ser. No. 773,381 to Lubowitz, Sheppard, and Torre). The linear arms of the oligomers contain conductive linkages, such as Schiff base (--N═CH--) linkages, between aromatic groups. Sulfone and ether linkages are interspersed in the arms. Each arm is terminated with a mono- or difunctional end cap (i.e., a radical having one or two crosslinking sites) to allow controlled crosslinking upon heat-induced or chemically-induced curing.

Polyamides of this same general type are described in our patent application U.S. Ser. No. 061,938 (now U.S. Pat. No. 4,876,328); polyetherimides, in U.S. Ser. No. 016,703 (now U.S. Pat. No. 4,851,495); and polyamideimides, in U.S. Ser. No. 092,740 (now abandoned).

SUMMARY OF THE INVENTION

High performance, aerospace advanced composites can be prepared using crosslinkable, end-capped polyester sulfone oligomers of the present invention that have a desired blend of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. The linear polyester oligomers include compounds selected from the group consisting of: ##STR1## and are prepared by the condensation of acid halides (or acids) and dialcohols (bisphenols), wherein:

E=a crosslinkable (unsaturated) end cap to improve the solvent resistance of the cured oligomer in the advanced composite;

A=a residue of a dicarboxylic acid halide; and

B=a residue of a dialcohol.

Generally, A and B are each linear aromatic moieties that include electronegative ("sulfone") linkages between aromatic radicals. Suitable electronegative linkages are selected from --SO₂ --, --S--, --O--, --CO--, --(CH₃)₂ C--, and --(CF₃)₂ C--. Schiff base (--CH═N--), oxazole, thiazole, and imidazole linkages can also be used in the backbone where semiconductive or conductive advanced composites are desired, since these linkages are conductive or semiconductive when suitably doped. Preferred Schiff base oligomers have the general formula: ##STR2## wherein R=an aromatic moiety or a short aryl chain including a plurality of aryl moieties linked with any of --CH₂ --, --SO₂ --, --S--, --O--, --CO--, --(CH₃)₂ C--, or --(CF₃)₂ C--; and q=0-4.

The crosslinkable end caps (E) are usually imidophenols or phenylimide acid halides that include a radical selected from the group consisting of: ##STR3## wherein R₁ =lower alkyl, lower alkoxy, aryl, aryloxy, substituted alkyl, substituted aryl, halogen, or mixtures thereof;

j=0, 1, or 2;

G=--CH₂ --, --O--, --S--, or --SO₂ --;

T=methallyl or allyl; and

Me=methyl.

Multidimensional oligomers include an aromatic hub and three or more arms wherein each arm includes an ester linkage and an end cap.

Blends of the linear or multidimensional oligomers comprise mixtures of the oligomers and one or more compatible, noncrosslinking polymers, and may be interpenetrating networks (IPNs).

The oligomers and blends can be processed with suitable reinforcing additives in continuous or chopped form to make prepregs. Upon curing, the composites are relatively thermally stable, tough, and solvent resistant.

The oligomers are prepared by simultaneously condensing the end caps, dicarboxylic acid halides, and dialcohols, (and hubs, if making multidimensional oligomers) in a suitable solvent under an inert atmosphere.

BEST MODE CONTEMPLATED FOR MAKING AND USING THE PRESENT INVENTION

Linear polyester oligomers of the present invention have the general formula: ##STR4## wherein E=a crosslinkable end cap radical preferably selected from the group consisting of: ##STR5## wherein R₁ =lower alkyl, lower alkoxy, aryl, aryloxy, substituted alkyl, substituted aryl, halogen, or mixtures thereof;

j=0, 1, or 2;

G=--CH₂ --, --O--, --S--, or --SO--₂ ;

T=methallyl or allyl;

Me=methyl;

A=the residue of a dicarboxylic acid halide (i.e. diacid halide);

B=the residue of a dialcohol; and

m=an integer, usually between 0 and 50.

The condensation can occur between alcohol (phenol) and acid halide functionalities or the corresponding acid or phenate groups.

The linear oligomers are usually prepared by reacting:

(a) 2 moles of a crosslinkable end-cap acid halide of the general formula (E.paren close-st._(i) .O slashed.--COX;

(b) n moles of an aromatic diacid halide or of a difunctional chain including a plurality of aryl rings linked with at least one linkage selected from the group consisting of --SO₂ --, --O--. --S--, --CO--, --(CH₃)₂ C--, --(CF₃)₂ C--, or mixtures thereof throughout the chain, the chain having an acid halide functionality on each end; and

(c) (n+1) moles of an aromatic dialcohol having terminal --OH functionalities,

or by reacting:

(a) 2 moles of a crosslinkable end-cap phenol of the general formula (E.paren close-st._(i) .O slashed.--OH;

(b) (n+1) moles of an aromatic diacid halide or of a difunctional chain including a plurality of aryl rings linked with at least one linkage selected from the group consisting of --SO₂ --, --O--, --S--, --CO--, --(CH₃)₂ C--, --(CF₃)₂ C--, or mixtures thereof throughout the chain, the chain having an acid halide functionality on each end; and

(c) n moles of an aromatic dialcohol (diol) having terminal --OH functionalities.

The diacid halide (or dicarboxylic acid, if desired) generally includes an aromatic radical selected from the group consisting of:

(a) phenyl; (b) naphthyl; (c) biphenyl;

(d) a polyaryl "sulfone" divalent radical of the general formula: ##STR6## wherein D=--S--, --O--, --CO--, --SO₂ --, --(CH₃)₂ C--, --(CF₃)₂ C--, or mixtures thereof throughout the chain;

(e) a divalent radical having conductive linkages, illustrated by Schiff base compounds selected from the group consisting of: ##STR7## wherein R is selected from the group consisting of:

phenyl; biphenyl; naphthyl; or

a divalent radical of the general formula: ##STR8##

wherein W=--SO₂ -- or --CH₂ --; and q=0-4; or

(f) a divalent radical of the general formula: ##STR9##

wherein R¹ =a C₂ to C₁₂ divalent aliphatic alicyclic, or aromatic radical, and, preferably, phenyl (as described in U.S. Pat. No. 4,556,697).

The preferred diacid halides include: ##STR10##

Schiff base acid halides can be prepared by the condensation of aldehydes and amines in the general reaction scheme: ##STR11## or similar schemes.

Thiazole, oxazole, or imidazole linkages, especially between aryl groups, may also be used in the conductive or semiconductive oligomers along with or in place of the Schiff base linkages.

Other diacid halides that can be used, but that are not preferred, are disclosed in U.S. Pat. No. 4,504,632, and include: adipylchloride,

malonyl chloride,

succinyl chloride,

glutaryl chloride,

pimelic acid dichloride,

suberic acid dichloride,

azelaic acid dichloride,

sebacic acid dichloride,

dodecandioic acid dichloride,

phthaloyl chloride,

isophthaloyl chloride,

terephthaloyl chloride,

1,4-naphthalene dicarboxylic acid dichloride, and

4,4'-diphenylether dicarboxylic acid dichloride,

Polyaryl or aryl diacid halides are preferred to achieve the highest thermal stabilities in the resulting oligomers and composites insofar as aliphatic bonds are not as thermally stable as aromatic bonds. Particularly preferred compounds include intermediate electronegative ("sulfone") linkages to improve toughness of the resulting oligomers. For purposes of this description, "sulfone" linkages include --SO₂ --, --S--, --CO--, and --(CF₃)₂ C--, unless clearly limited to only --SO₂ --.

The dialcohol (i.e. diol, dihydric phenol, or bisphenol) is generally selected from the group consisting of:

HO--Ar--OH;

HO--Ar--L--Ar'--L--Ar--OH;

HO--Ar'--L--Ar--L--Ar'--OH;

wherein L=--CH₂ --, --(CH₃)₂ C--, --(CF₃)₂ C--, --O--, --S--, --SO₂ --, or --CO--; ##STR12## T and T₁ =lower alkyl, lower alkoxy, aryl, aryloxy, substituted alkyl, substituted aryl, halogen, or mixtures thereof;

q=0-4;

k=0-3; and

j=0, 1, or 2,

The dialcohols also include hydroquinone; bisphenol-A; p,p'-biphenol; 4,4'-dihydroxydiphenylsulfide; 4,4'-dihydroxydiphenylether; 4,4'-dihydroxydiphenylisopropane; 4,4'-dihydroxydiphenylhexafluoropropane; a dialcohol having a Schiff base segment, the radical being selected from the group consisting of: ##STR13## wherein R is selected from the group consisting of:

phenyl;

biphenyl;

naphthyl; or

a radical of the general formula: ##STR14##

wherein W=--CH₂ -- or --SO₂ --; or

dialcohol selected from the group: ##STR15## wherein L is as defined above;

Me=methyl;

m=an integer, generally less than 5, and preferably 0 or 1; and

D=any of --CO--, --SO₂ --, or --(CF₃)₂ C--.

While bisphenol-A is preferred in the etherimide synthesis (because of cost and availability), the other dialcohols can be used to add rigidity to the oligomer without significantly increasing the average formula weight, and, therefore, can increase the solvent resistance. Random or a block copolymers are possible.

Furthermore, the dialcohols may also be selected from the those described in U.S. Pat. Nos. 4,584,364; 3,262,914; or 4,611,048.

Dialcohols of this nature are commercially available. Some may be easily synthesized by reacting halide intermediates with bis-phenates, such as by the reaction of 4,4'-dichlorodiphenylsulfone with bis(disodium biphenolate).

Schiff base diols are prepared by the condensation of aldehydes and amines under the general reaction schemes: ##STR16##

Polyester Schiff base compounds are then prepared by condensing the Schiff base diols with end cap acid halides (or carboxylic acids) and diacid halide chains.

Schiff base acid halides can be prepared by the general reaction scheme: ##STR17## or similar schemes as with the diols. The resulting polyesters are made by condensing the acid halides with suitable diols.

While illustrated for preparing Schiff base diacid halides and diols, these reaction schemes are representative of the processes used to prepare any of the diacid halides or diols used in the polymerization reactions. Inexpensive, commercially available starting compounds are used, such as aminophenol.

Dopants for creating semiconductive or conductive composites are generally selected from two of the three categories commonly used to dope other polymers, namely (I) dispersions of alkali metals (for high activity) or (II) strong chemical oxidizers, particularly alkali perchlorates (for lower activity). Arsenic compounds and elemental halogens, while active dopants, are too dangerous for general usage, and are not recommended.

The dopants react with the polymers to form charge transfer complexes. N-type semiconductors result from doping with alkali metal dispersions. P-type semiconductive result from doping with elemental iodine or perchlorates.

While research into conductive or semiconductive polymers has been intense, the resulting compounds, mainly polyacetylenes, polyphenelenes, and polyvinylacetylenes, are unsatisfactory for aerospace applications because the polymers are:

(a) unstable in air;

(b) unstable at high temperatures;

(c) brittle after doping;

(d) toxic because of the dopants; or

(e) intractable.

The polymers of the present invention exhibit greater oxidative stability and greater dimensional stability at elevated temperatures, greater impact strengths and greater dimensional stiffness over a wider range of temperatures, and toughness over those oligomers of the prior art.

While conventional theory holds that semiconductive polymers should have low ionization potentials, long conjugation lengths, and planar backbones, there is an inherent trade-off, between conductivity and toughness or ease of processing, if these constraints are followed. To overcome the processing and toughness shortcomings common with Schiff base, polyoxazole, polyimidazole, and polythiazole polymers, in the present invention, the polymers include mechanical swivels interspersed along the backbone coupling the rigid, conductive ribbons. Thermally stable swivels are afforded through "sulfone" linkages, particularly in phenoxyphenylsulfone or phenoxyphenylketone moieties.

The conductive, semiconductive, and nonconductive polymers are usually made by mixing the appropriate proportions of the reactants in a polar organic solvent at temperatures between about -30° to 100° C. Stepwise synthesis may also be used, if desired, as outlined for the Schiff base oligomers.

Because the aromatic polyester sulfone resins synthesized in accordance with this invention have appreciable molecular weight between the reactive groups, the oligomers will retain sufficient plasticity to be processible during fabrication prior to crosslinking of the end caps to thermoset composites. If possible, thermoplastic formulations with even higher molecular weights should be synthesized. The polyesters preferably have MWs between about 5000-40,000, and, more preferably, between about 15,000-25,000.

Because it may be difficult to make diols or diacid chlorides having intermediate benzimidazole, benzoxazole, or benzothiazole linkages, and because the chemistry for Schiff base compounds is well understood, it is preferred to use Schiff base segments in the oligomers for semiconductive or conductive applications.

Solubility of the oligomers becomes an increasing problem as the length of the chains increases. Therefore, shorter chains are preferred, if the resulting oligomers remain easily processed. That is, the chains should be long enough to yield thermoplastic characteristics but short enough to keep the oligomers soluble during the reaction sequence.

Blends can improve impact resistance of composites prepared from the crosslinkable oligomers without causing a significant loss of solvent resistance. The blends comprise mixtures of one or more crosslinkable oligomers and one or more polymers that are incapable of crosslinking, and generally comprise substantially equimolar amounts of one polymer and an oligomer having substantially identical backbones. The crosslinkable oligomer and comparable polymer can be blended together by mixing mutually soluble solutions of each. While the blend is preferably equimolar in the oligomer and polymer, the ratio of the oligomer and polymer can be adjusted to achieve the desired physical properties.

Although the polymer in the blend usually has the same type of backbone (i.e. polyester sulfone) and same length backbone as the oligomer, the properties of the composite formed from the blend can be adjusted by altering the ratio of formula weights for the polymer and oligomer. The oligomer and polymer generally have substantially identical repeating units, but the oligomer and polymer merely need be compatible in the solution prior to sweeping out as a prepreg. Of course, if the polymer and oligomer have identical backbones, compatibility in the blend is likely to occur.

Quenching compounds can be employed, if desired to regulate the polymerization of the comparable polymer, so that it has an average formula weight substantially identical with the crosslinkable oligomer. For thermal stability, an aromatic quenching compound, such as phenol or benzoic acid chloride, is preferred.

The noncrosslinking polymer can be made by the same synthetic method as the oligomer with the substitution of a quenching cap for the crosslinking end cap.

While the best blends are probably those in which the backbones are essentially identical and of modest formula weight and those in which the oligomer and polymer are in equimolar proportions, other compositions may be prepared, as will be recognized by those of ordinary skill in the art.

Solvent resistance may decrease markedly if the comparable polymer is provided in large excess to the crosslinkable oligomer in the blend.

As discussed, the blends will generally comprise a mixture of one oligomer and a polymer of the same type. The polymer may, however, be from another family, such as an ester, imide, ether, or amide. The mixture may include multiple oligomers or multiple polymers, such as a three component mixture of oligomers and polymers. Other combinations will be recognized by those of ordinary skill in the art.

The blends may be semi-interpenetrating networks of the general type described by Egli et al., "Semi-Interpenetrating Networks of LARC-TPI" available from NASA-Langley Research Center.

Multidimensional polyesters (of the general formula: Ar.paren open-st.Q)_(w), wherein Ar=an aromatic radical of valence w, Q=a polyester radical having a crosslinking end cap and w is an integer greater than or equal to 3) can also be prepared by linking polyester sulfone or polyester arms to an aromatic hub. The multidimensional oligomers, then, have three or more chains radiating from the hub like rays or spokes. Each chain includes a crosslinking end-cap radical at its distal end to provide one or two crosslinking sites. Accordingly, the multidimensional oligomers have a greater number (i.e. higher density) of crosslinking bonds when cured than the linear oligomers.

Usually the hub will have three radiating chains to form a "Y" pattern. In some cases, four chains may be used. Including more chains leads to steric hindrance as the hub is too small to accommodate the radiating chains. A trisubstituted phenyl hub (such as phloroglucinol) is highly preferred with the chains being symmetrically placed about the hub. Biphenyl, naphthyl, or azaline (e.g., melamine) may also be used as the hub radical along the other aromatic moieties, if desired.

Triazine derivatives described in U.S. Pat. No. 4,617,390 (or the corresponding acid halides) can be used as the hub. Other suitable triazine derivatives (described in U.S. Pat. No. 4,574,154) that are suitable hubs can be made by reacting a compound of the general formula: ##STR18## wherein R₂ is a divalent hydrocarbon residue containing 1-12 carbon atoms (and, preferably, ethylene) with phthalic acid anhydride to form arms that include imide linkages and terminal acid functionalities (that can be converted to acid halides, if desired).

Other acid halide (or carboxylic acid) hubs can be made by reacting polyol aromatic hubs, such as phloroglucinol, with nitrobenzoic acid or nitrophthalic acid to form ether linkages and active, terminal carboxylic acid functionalities. The nitrobenzoic acid products would have three active sites while the nitrophthalic acid products would have six; each having the respective formula:

    .O slashed..brket open-st.O--.O slashed.--COOH!.sub.3 or .O slashed..brket open-st.O--.O slashed..paren open-st.COOH).sub.2 !.sub.3

wherein, .O slashed.=phenyl. Of course other nitro/acids can be used.

Hubs can also be formed by reacting the corresponding halo-hub (such a tribromobenzene) with aminophenol to form triamine compounds represented by the formula: ##STR19## which can then be reacted with an acid anhydride to form a polycarboxylic acid of the formula: ##STR20## wherein, .O slashed.=phenyl; the hub being characterized by an intermediate ether and imide linkage connecting aromatic groups. Thio-analogs are also contemplated, in accordance with U.S. Pat. No. 3,933,862.

Phenoxyphenyl sulfone arms radiating from a hub with a terminal amine, carboxylic acid, or hydroxyl group are also precursors for making multidimensional polyester oligomers of the present invention.

The best results are likely to occur when the hub is phloroglucinol or cyuranic acid. In either case a suitable end-cap monomer (phenol or acid halide) can be reacted with the hub to form "short-armed," multidimensional oligomers having three or six crosslinking sites. These compounds are the simplest multidimensional oligomers and are relatively inexpensive to synthesize.

The multidimensional oligomers are generally prepared by simultaneously reacting the hub with dialcobols; diacid halides; dialcohols and diacid halides; and suitable end-cap monomers under reaction conditions similar to those for the linear oligomers. Care should be taken, however, to reduce the likelihood of undesirable side reactions. For example, a mixture of about 1 mole of the hub, w moles of dialcohol, w moles of diacid halide, and w moles of end-cap monomer is likely to form a complex mixture of multidimensional and linear oligomers and polymers. Accordingly, staged or stepwise reaction of the several reactants is preferred in this synthesis.

Thus, the oligomers can be synthesized in a homogeneous reaction scheme wherein all the reactants are mixed at one time, or in a stepwise reaction scheme wherein the radiating chains are affixed to the hub and the product of the first reaction is subsequently reacted with the end cap groups. Of course, the hub may be reacted with end-capped arms that include one reactive, terminal functionality for linking the arm to the hub. Homogeneous reaction is preferred, resulting undoubtedly in a mixture of oligomers because of the complexity of the reactions. The products of the processes (even without distillation or isolation of individual species) are preferred oligomer mixtures which can be used without further separation to form advanced composites.

Blends of the multidimensional oligomers, comparable to the blends of linear oligomers, can also be prepared, as will be understood.

The preferred aromatic moieties in the linear or multidimensional oligomers are aryl groups, such as phenyl, biphenyl, and naphthyl. Other aromatic groups can be used, if desired, since their stabilized aromatic bonds should provide the desired thermal stability. For example, azaline groups may be used. As discussed earlier, the aryl groups may include substituents, if desired, such as halogen, lower alkyl up to about 4 carbon atoms, lower alkoxy up to about 4 carbon atoms, or aryl side chains. Steric hindrance may arise in synthesizing the oligomers or in crosslinking the oligomers into cured composites, if the side chains are too large. The substituents may also effect the thermal stability of the resulting oligomers and composites. Unsubstituted phenyl groups are preferred for cost, convenience, and performance. The m-isomer may be preferred, since it tends to create a folded ribbon structure in the backbone.

Prepregs of the oligomers or blends can be prepared by conventional techniques. While woven fabrics are the typical reinforcement, the fibers can be continuous or discontinuous (in chopped or whisker form) and may be ceramic, organic, carbon (graphite), or glass, as suited for the desired application.

Composites can be formed by curing the oligomers or prepregs in conventional vacuum bag techniques. The oligomers can also be used as adhesives, varnishes, films, or coatings.

While preferred embodiments have been described, those skilled in the art will readily recognize alterations, variations, and modifications which might be made without departing from the inventive concept. Therefore, the claims should be interpreted liberally with the support of the full range of equivalents known to those of ordinary skill on the basis of this description. The claims should be limited only as is necessary in view of the pertinent prior art. 

We claim:
 1. A method for making a polyester oligomer that is suitable for advanced composites comprising the step of:condensing simultaneously under an inert atmosphere in a suitable solvent a mixture comprising about: (i) 2 moles of a crosslinking end-cap phenol of the formula:

    D.sub.i --.O slashed.--OH

wherein i=1 or 2;.O slashed.=phenyl; D= ##STR21## R₁ =lower alkyl, lower alkoxy, aryl, aryloxy, substituted alkyl substituted aryl, halogen, or mixtures thereof; j=0, 1, or 2; G=--CH₂ --, --O--, --S-- or --SO₂ --; T=methallyl or allyl; and Me=methyl; (ii) (n+1) moles of a dicarboxylic acid halide; and (iii) n moles of a dialcohol, wherein n is an integer greater than or equal to
 1. 2. A method for making a polyester oligomer that is suitable for advanced composites comprising the step of:condensing simultaneously under an inert atmosphere in a suitable solvent a mixture comprising about: (i) 2 moles of a crosslinking end-cap acid halide of the formula:

    D.sub.i --.O slashed.--COX

wherein i=1 or 2;.O slashed.=phenyl; D= ##STR22## R₁ =lower alkyl, lower alkoxy, aryl, aryloxy, substituted alkyl, substituted aryl, halogen, or mixtures thereof; j=0, 1, or 2; G=--CH₂ --, --O--, --S--, or --SO₂ --; T=methallyl or allyl; and Me=methyl, (ii) n moles of a dicarboxylic acid halide; and (iii) (n+1) moles of a dialcohol, wherein n is an integer greater than or equal to
 1. 